Many valves, so-called point-of-use valves, used in pharmaceutical piping applications, are valves which are operated manually. Because systems in which these valves are used are periodically sterilized with steam, resulting in safety concerns for valve operators, and because many systems are now required to know the position of valves at all times, great emphasis is placed on indicating the valve position of manual valves, and on providing automatic means for correctly locating and locking out the relevant valves without manual intervention. A temperature-responsive locking mechanism for a manual valve is described in commonly assigned U.S. Pat. No. 5,462,226 issued Oct. 31, 1995, Temperature-Responsive, Locking Mechanism For, And In Combination With, A Fluid Valve, incorporated herein by reference. Until now, two methods have primarily been used to indicate valve position on a manual valve. In the first method, visual indication is used to identify whether a valve is open or closed. Here, bright colors are attached to a part of the valve which moves as the valve is opened or closed in order to identify what position the valve is in. This mechanical method, however, does not provide an operator precise knowledge of the valve position as the valve strokes, other than whether the valve is fully open or fully closed. Under the second method, electronic signals are sent to a programmable logic controller or digital control system by means of a mechanical or proximity switch located at either the open position, the closed position, or at each position to indicate the valve state. For mechanical switches, the switch sends a signal when it is tripped; proximity switches transmit a signal corresponding to an open or closed valve position when a conductor moves into the switch sensing area. This method also suffers the disadvantage of being unable to indicate continuous valve position as the valve strokes. Moreover, standard switches cannot limit the opening of a manual valve, but only provide an electrical output indicating that the valve is open or closed. In addition, because manual valves are often located in environments either open to the natural elements such as rain and snow, or requiring regular washdowns such as in a sanitary facility, a switch cannot simply be attached to the exterior of a manual valve, but must be located in a NEMA (National Electrical Manufactures Association) enclosure. A terminal connection must be used to provide electrical power to the switch, also in the NEMA enclosure, thus making the switch even larger and more difficult to attach to a valve.
Consequently, a compact means for providing an indication of valve position throughout the entire valve stroke, for providing an electronic manual override of a locked valve, and for preventing a valve from fully opening at any position throughout the valve stroke is greatly desired.